Introduction to Virology

t/f viruses are alive

trick question- they are obligate intracellular organisms whose life cycle is intimately tied to that of its host cell, they cannot make energy or macromolecules (DNA/RNA/proteins) independently of the eukaryotic host cell

why is virology relevant to dentistry?

dental infection control

patient screening, personal hygiene, personal protection, instrument processing, surface asepsis, patient treatment, laboratory disinfection

what are the various methods of classifying viruses and what are the classifications within that type?

type of nucleic acid genome: DNA or RNA / type of strand and their polarity / segmented or one piece

capsid: icosahedral or helical or complex / enveloped or non-enveloped

replication strategy

genome sequence similarity

symptoms: hepatitis virus, respiratory virus, encephalitis virus

tropism: neurotropic, ect

mode of transmission: arboviruses

host: plant viruses, animal viruses, bacteriophage

virion

complete virus particle

nucleic acid core

DNA or RNA, nucleoproteins

protein coat

capsid, nucleocapsid, spikes/viral attachment proteins (VAPs)

virion

a virus particle, nanometers (nm) are unit of measurement

what is the clinically important range for viruses?

18nm - 300 nm

capsid

single or double layer protein shell surrounding the viral nucleic acid, composed of subunits (protomer and capsomer) arranged in a symmetric pattern

nucleocapsid

nucleic acid + capsid

each capsid subunit has the capacity to…

bind to other subunits in specific ways, these physical interactions between subunits permits self assembly to form virus capsid (virions)

spikes / viral attachment proteins

surface structures that mediate the interaction of the virus with the target cell, removal or disruption inactivates the virus, antibodies generated against it prevent virus infection

enveloped virus

lipid bi-layer envelope, protein projections like glycoproteins or VAPs, is essential for infectivity

naked capsid characteristics

stable, released by cell lysis, transmission: easily spread, fomites, longer survival time, may survive gut

enveloped virus characteristics

labile, released by budding or cell lysis, transmission: must remain wet, close contact or droplets, destroyed in gut

what are some structures of capsids

icosahedral, helical, complex

helical symmetry

example is TMV, non-enveloped, capsid: RNA helix with associated nucleoproteins

icosahedral symmetry

repeating subunits (few proteins), subunits can self assemble, efficient (largest volume possible), very strong

T4 bacteriophage

both helical and icosahedral structure

complex virus structures

general principles of symmetry are often used to build part of the virus shell but it cannot be simply defined by a mathematical equation

poxvirus

complex symmetry

DNA in viruses

stable molecule, larger (3-300kbp), dsDNA (circular and linear), pdsDNA (partially double/single stranded circular DNA), ssDNA (linear)

RNA in viruses

easily degraded, smaller (3-30kbp), dsRNA (segment), +ssRNA, -ssRNA, -ssRNA segmented, +_ ambisense

t/f the viral genome contains all the information needed to direct the host cell to synthesize virus encoded proteins but they are dependent on the host cell for substrates, energy, and machinery required for protein and nucleic acid synthesis

true

t/f in most cases, host cells mcaromolecular synthesis is not shut down, the host cells just make their proteins along with viral proteins

false- most of the time viruses shut down host cell macromolecular synthesis

what are the general steps of the replication cycle of all viruses

adsorption or attachment

penetration

uncoating and eclipse

transcription

synthesis of viral components

assembly

release of virions

what steps may overlap?

penetration and uncoating

where do critical events of viral replication occur?

cell nucleus or exclusively within the cytoplasm

how long is the period between infection and the production of the new virion?

can be as short as 3 hrs or as long as several months to years

adsorption or attachment

VAPs mediate recognition and attachment of the virus to receptors on host cells

host range, cell, and tropism may be restricted by specificity of VAP for host cell receptors expressed only on certain cell types

penetration

use the same pathways used by cells to take up macromolecules

non-enveloped: receptor mediated endocytosis (virion is taken up into endosomes) or viropexis (virion penetrates plasma membrane directly)

enveloped: fusion and release the nucleocapsid or genome into the cytoplasm, viral proteins mediate fusion with cellular membranes

uncoating and eclipse

period where no intact infectious virus can be detected, begins with uncoating of lipid membrane and protein capsid surrounding the nucleic acid viral core, as uncoating proceeds the viral nucleic acid becomes free to act as a template for synthesis of virus RNA

transcription

virus mRNA codes for synthesis of enzymes necessary to complete the process of uncoating itself and also initiate early steps in viral replication

during this, the synthesis of host cell RNA is halted so host ribosomes are free to receive viral mRNA and provide a focus for transcription and synthesis of viral proteins

structural viral proteins

proteins that make up the virus particle, synthesized on cellular polyribosomes

non-structural viral proteins

enzyme required for virus genome replication

t/f during synthesis of viral components, there is a simultaneous synthesis of progeny viral nucleic acid using synthesized nucleic acid polymerases

true

assembly

accomplished by incorporation of viral nucleic acid into putative capsomeres: procapsids, may occur in the cell nucleus, cytoplasm, or at the plasma membrane (enveloped viruses)

release of virions

may occur either through gradual budding (enveloped viruses) or sudden rupture

viral genetics

mutations and the effects on viral replication and pathogenesis, interactions of 2 genetically different viruses when they co-infect the same cell

wild type / parental virus

the original virus from which mutants or variants are derived / compared to

may not be the same as the virus found in nature

what are the types of mutations?

spontaneous or induced

lethal mutation

mutations in essential genes, virus cannot replicate

attenuated mutations

mutations that result in less virulent virus strains or variants, often developed and used as vaccine strains

revertant mutant

mutation that results in a change from a mutant genotype to the wild type genotype

defective viruses

lack 1 or more functional genes required for virus production and require helper activity from another virus

some require unrelated helper viruses

recombination

interaction among viruses that results in progeny that have phenotypic and genotypic differences from either parent, typically occurs via nucleic acid strand breakage and recombination between parental genomes

reassortment

interaction among viruses that can occur with viruses with segmented genomes when 2 different virus strains co-infect a cell, packaging of gene segments from the different parental strains in progeny may result in new virus phenotype

complementation

interaction of viral gene products in cell infected with 2 viruses, 1 or both defective

allows replication of 1 or both viruses under normally non-permissive conditions

genotypes are unchanged

results from 1 virus providing gene product that is defective in the other virus

if both viruses are defective in the same gene, no complementation

phenotypic mixing / transcapsidation

can occur when cells co-infect with different virus strains and progeny virions are produced that have the genome from one strain, but the capsid proteins from the other strain